fmt.Scan: empty strings are errors

Fixes #1002.

R=rsc
CC=golang-dev
https://golang.org/cl/1882046

src/pkg/fmt/scan.go

@@ -598,18 +598,24 @@ func (s *ss) scanComplex(verb int, n int) complex128 {
 
 // convertString returns the string represented by the next input characters.
 // The format of the input is determined by the verb.
-func (s *ss) convertString(verb int) string {
+func (s *ss) convertString(verb int) (str string) {
 	if !s.okVerb(verb, "svqx", "string") {
 		return ""
 	}
 	s.skipSpace(false)
 	switch verb {
 	case 'q':
-		return s.quotedString()
+		str = s.quotedString()
 	case 'x':
-		return s.hexString()
+		str = s.hexString()
+	default:
+		str = s.token() // %s and %v just return the next word
 	}
-	return s.token() // %s and %v just return the next word
+	// Empty strings other than with %q are not OK.
+	if len(str) == 0 && verb != 'q' && s.maxWid > 0 {
+		s.errorString("Scan: no data for string")
+	}
+	return
 }
 
 // quotedString returns the double- or back-quoted string represented by the next input characters.

src/pkg/fmt/scan_test.go

@@ -464,6 +464,46 @@ func TestScanMultiple(t *testing.T) {
 	if a != 123 || s != "abc" {
 		t.Errorf("Sscan wrong values: got (%d %q) expected (123 \"abc\")", a, s)
 	}
+	n, err = Sscan("asdf", &s, &a)
+	if n != 1 {
+		t.Errorf("Sscan count error: expected 1: got %d", n)
+	}
+	if err == nil {
+		t.Errorf("Sscan expected error; got none", err)
+	}
+	if s != "asdf" {
+		t.Errorf("Sscan wrong values: got %q expected \"asdf\"", s)
+	}
+}
+
+// Empty strings are not valid input when scanning a string.
+func TestScanEmpty(t *testing.T) {
+	var s1, s2 string
+	n, err := Sscan("abc", &s1, &s2)
+	if n != 1 {
+		t.Errorf("Sscan count error: expected 1: got %d", n)
+	}
+	if err == nil {
+		t.Errorf("Sscan <one item> expected error; got none")
+	}
+	if s1 != "abc" {
+		t.Errorf("Sscan wrong values: got %q expected \"abc\"", s1)
+	}
+	n, err = Sscan("", &s1, &s2)
+	if n != 0 {
+		t.Errorf("Sscan count error: expected 0: got %d", n)
+	}
+	if err == nil {
+		t.Errorf("Sscan <empty> expected error; got none")
+	}
+	// Quoted empty string is OK.
+	n, err = Sscanf(`""`, "%q", &s1)
+	if n != 1 {
+		t.Errorf("Sscanf count error: expected 1: got %d", n)
+	}
+	if err != nil {
+		t.Errorf("Sscanf <empty> expected no error with quoted string; got %s", err)
+	}
 }
 
 func TestScanNotPointer(t *testing.T) {